Methods of fabricating electrochemical cells

ABSTRACT

Conditioning secondary lithium ion cells at elevated temperatures above ambient reduces the time required to complete this process and produces cells and batteries which demonstrate improved electrochemical performance. Conditioning includes subjecting an electrochemical cell to at least one full charge/discharge cycle whereby gases generated and removed before the cell is sealed and ready for use. The cell is placed in an environment that is maintained at a temperature of at least 30° C., charged and discharged, and sealed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application national stage application and claims benefit ofInternational Application No. PCT/US98/08972, which designates theUnited States, and has an international filing date of May 5, 1998,which claims priority to and is a continuation U.S. patent applicationSer. No. 08/857,025, which was filed on May 15, 1997 now U.S. Pat. No.5,871,865, which are both incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed to electrochemical devices and, moreparticularly to methods of fabricating a secondary lithium ion batterywhereby the battery is conditioned (i.e., charged and discharged) atelevated temperatures prior to being packaged and sealed for immediateuse or storage. The battery produces gases during the condition processwhich are removed.

BACKGROUND OF THE INVENTION

Electrochemical cells comprise a cathode, an anode, and a polymericmatrix or separator containing an electrolyte interposed therebetween.Non-aqueous lithium electrochemical cells are discussed in U.S. Pat.Nos. 4,472,487, 4,668,595, 5,028,500, 5,441,830, 5,460,904, and5,540,741.

The anode comprises a compatible anodic material which is any materialwhich functions as an anode in an electrochemical cell. Such compatibleanodic materials are well known in the art and include, by way ofexample, lithium, lithium alloys, such as alloys of lithium withaluminum, mercury, nickel, zinc, and the like, and intercalation basedanodes such as carbon, WO₃, and the like. The cathode comprises acompatible cathodic material which refers to any material whichfunctions as a positive pole (cathode) in an electrochemical cell. Suchcompatible cathodic materials are well known in the art and include, byway of example, manganese dioxide, molybdenum trioxide, sulfides oftitanium and niobium, chromium oxide, copper oxide, vanadium oxides suchas V₂O₅, V₆O₁₃, LiV₃O₈ and the like. The particular compatible cathodicmaterial employed is not critical. When the electrochemical cell is asecondary cell, then the compatible cathodic material employed is onewhich is capable of being recharged (e.g., LiV₃O₈, V₆O₁₃, MoO₃, and thelike).

Composite electrode refers to cathodes and anodes wherein the cathodeincludes materials in addition to compatible cathodic materials and theanode includes materials in addition to compatible anodic materials.Typically, the composite electrode contains a polymer which acts to bindthe composite materials together and an electrolytic solvent. Compositeelectrodes are well known in the art. For example, a composite cathodecan comprise a compatible cathodic material, a conductive material, anelectrolytic solvent, an alkali salt, and a matrix forming polymer.Similarly, for example, a composite anode can comprise a compatibleintercalation anodic material, an electrolyte solvent and a matrixforming polymer.

Secondary lithium ion cells and batteries employing composite electrodesare typically fabricated in the discharged state which means that theanode comprises intercalation carbon materials and the cathode comprisesa suitable lithiated cathodic material, e.g., lithiated manganese oxide.Prior to use, the cell must be charged with external energy so thatlithium ions from the cathodic material are intercalated into the carbonmaterial of the anode. It has been found that during the initialcharge/discharge cycles the cell generates a considerable amount ofgases. These gases would be entrapped in the cell unless they areremoved by a conditioning process prior to sealing the package encasingthe cell.

SUMMARY OF THE INVENTION

The present invention is based, in part, on the discovery thatconditioning secondary lithium ion cells at elevated temperaturesreduces the time required to complete this process. Specifically, theinventive process improves the efficiency at which gas is produced andremoved from the cells. In addition, this process produces cells andbatteries which demonstrate improved electrochemical performance.

In one aspect, the invention is directed to a method of preparing anelectrochemical cell the includes the steps of:

(a) fabricating an electrochemical cell in the discharged state whichcomprises (i) an anode comprising an intercalation based carbonmaterial, an (ii) a cathode comprising a lithiated cathodic material,and (iii) a polymeric matrix or separator interposed between the anodeand cathode which comprises an electrolyte solvent and salt;

(b) placing the electrochemical cell in an environment that ismaintained at an elevated temperature;

(c) charging and discharging the electrochemical cell;

(d) removing gas that is generated by the electrochemical cell in step(c); and

(e) sealing the electrochemical cell.

In another aspect, the invention is directed to a method of preparing anelectrochemical cell that includes the steps of:

(a) preparing an anode precursor by forming an anode film comprising ananodic material, a first polymeric binder, and a first plasticizer andthereafter removing said first plasticizer;

(b) preparing a cathode precursor by forming a cathode film comprising acathodic material, a second polymeric binder and thereafter removingsaid third plasticizer;

(c) positioning a polymer electrolyte precursor between said anode filmand said cathode film to form an electrochemical cell precursor andactivating the electrochemical cell precursor to form an electrochemicalcell;

(d) placing the electrochemical cell in an environment that ismaintained at an elevated temperature;

(e) charging and discharging the electrochemical cell;

(f) removing gas that is generated from the electrochemical cell in step(e); and

(g) sealing the electrochemical cell.

In a further aspect, the invention is directed to a method of preparingan electrochemical cell that includes the steps of:

(a) forming an anode film comprising an anodic material, a polymericmatrix and a first plasticizer;

(b) forming a cathode film comprising a cathodic material, a polymericbinder and a second plasticizer;

(c) forming a polymeric or separator layer comprising a thirdplasticizer;

(d) interposing said polymeric or separator layer between said anodefilm and said cathode film;

(e) removing said plasticizers to form an electrochemical cellprecursor;

(f) activation said electrochemical cell precursor to form anelectrochemical cell;

(g) charging and discharging the electrochemical cell while theelectrochemical cell is maintained at an elevated temperature;

(h) removing gas that is generated from the electrochemical cell duringstep (f); and

(i) sealing the electrochemical cell.

Preferably, the elevated temperature ranges from about 25° C. to about80° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first cycle EVS voltage for electrochemical cells.

FIGS. 2A and 2B are graphs of coulombic efficiency vs. cycle anddischarge capacity vs. cycle, respectively, for electrochemical cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is directed to methods of preparingelectrochemical devices, and particularly to processes for fabricatingbatteries. Preferred electrochemical cells include: a composite cathodecomprising an active material and polymeric binder, a composite anodecomprising an intercalation based carbon anode and polymeric binder,with each electrode capable of reversibly incorporating (e.g.,intercalating) an alkali metal ion, and comprising a polymeric matrix orseparator containing an electrolyte solution comprising an organicelectrolyte solvent and a salt of the alkali metal. Particularlypreferred electrochemical cells and batteries use lithium and saltsthereof.

The anode generally comprises an anode film that is laminated onto oneor both sides of the current collector. Similarly, the cathode comprisesa cathode film that is laminated onto one or both sides of the currentcollector. The current collectors comprise, for example, a screen, grid,expanded metal, foil, woven or non-woven fabric or knitted wire formedfrom an electron conductive material such as metals or metal alloys.

A critical aspect of the present invention is that during fabrication,the electrochemical cell or battery is subject to a conditioning cyclingprocess whereby gases are released. After removal of these gases, theelectrochemical ell or battery is sealed for immediate use or storage.This conditioning process takes place at an elevated temperature whichreduces the number of cycles required to release and remove the gas. Inaddition, conditioning at elevated temperatures improves cellperformance, notably cycling, elevated temperature storage, ambienttemperature storage and reduction of secondary gassing. Secondarygassing is defined as residual gassing in a cell after the cell has beenconditioned, degassed, and then re-sealed. This phenomenon is believedto arise from the small amount of residual moisture or other impuritiesleft in the cell following condition.

The conditioning process comprises at least one charge/discharge cycleafter the electrochemical cell or battery is assembled in the dischargedstate. The number of cycles employed is not critical although the numbershould be sufficient to produce enough gas so that after the cell orbattery is sealed, no significant secondary gassing occurs. Generally,during the conditioning process, the amount of gas produced decreaseswith each cycle. Typically, by conditioning at elevated temperatures,only 1-2 cycles are required. The conditioning can also terminate at acharged state. In a preferred embodiment, conditioning comprises onecomplete charge/discharge cycle followed by a final charge half cycle.As is apparent, after conditioning at least one full cycle, theconditioning can then terminate at any state between compete charge andcompete discharge.

Conditioning is generally conducted at an elevated temperature that ishigher than ambient that typically ranges from about 25° C. to 80° C.,more preferably 30° C. to 60° C., and most preferably 35° C. to 50° C.and is otherwise accomplished by conventional methods. Conventionalcharging schemes can be employed including, for example, constantcurrent/constant voltage and pulsed current. A preferred technique is aconstant current, limited charge to an upper voltage followed by aconstant voltage charge, terminated either by time and/or minimum chargecurrent value, which is described in the Proceeding of the Third AnnualPortable by Design Conference, 1996, pp 13-22 (Penton Publishing). Theelectrochemical cell is typically charged (or recharged) at about 1 mAper cm². Similarly, the electrochemical cell is discharged at about 1 mAper cm². These rates will depend on a number of factors including, forexample, cell chemistry and the anode/cathode mass ratio. It should benoted that the enhanced kinetics effected by the elevated temperaturesmean that a higher mA per cm² charge/discharge rate could be used (e.g.,about 3 mA per cm²). This reduces the time required for conditioning.With the inventive technique, the charge and discharge current densitiescould be about 4 mA per cm² or higher.

A secondary lithium ion electrochemical cell having a graphite anode andLi_(x)MnO₄ cathode material typically has an initial charged potentialof at least about 4.2 volts and discharge is generally continued untilthe potential of the cell is reduced by about 3.0 volts. The voltagelimits will vary depending, for example, on the particular anode and/orcathode active materials. As is apparent, batteries will have differentinitial changed potential depending on the number of cells and how thecells are arranged. With the inventive technique, the charge anddischarge current can be as high as about 4 mA per cm².

However, prior to describing this invention in further detail, thefollowing terms will be defined.

The term “cycle” refers to a combined charge one-half cycle and adischarge one-half cycle, whereby the cell or battery takes in andstores electrical energy in a charge one-half cycle and releaseselectrical energy in a discharge one-half cycle.

The term “charge capacity” refers to the maximum charge measured inampere hours, which the cell or battery fabricated by the inventivetechnique is capable of achieving under the charging conditions andprocedures.

The term “cycle life” is the number of cycles undergone by the cellunder charging conditions and procedures and repeatedly discharged understandard test conditions and procedures, until the charge capacity ofthe cell or battery has fallen to one-half the charge capacity.

The term “initial charge capacity” refers to the charge capacity of afresh cell or battery which is fabricated by the inventive technique.

The term “standard cycle life” is the number cycles undergone by a freshcell or battery which is repeatedly slowly charged at a constant lowcurrent and repeatedly discharged under standard test conditions andprocedures, until the charge capacity of the cell or battery falls toone-half the initial charge capacity.

The term “plasticizer” refers to an organic solvent, with limitedsolubility of polymers, that facilitates the formation of porouspolymeric structures. By “porous structure” is meant that uponextraction of the plasticizer the polymer remains as a porous mass.Suitable plasticizers have high boiling points typically from about 100°C. to about 350° C. A number of criteria are important in the choice ofplasticizer including compatibility with the components of theelectrochemical cell precursor, processability, low polymer solubilityand extractability by dense gases. Preferred plasticizers include, forexample, dibutyl phthalate, dioctylphthalate, acetates, glymes, and lowmolecular weight polymers.

In operation, in fabricating a polymeric matrix or composite electrodethat includes polymeric binders, for example, the plasticizer is firstwell mixed with a polymer. Preferably the weight ratio of plasticizer topolymer in this mixture is from about 0.1 to about 50, more preferablyabout 0.5 to about 25, and most preferably about 1 to about 10.Thereafter the plasticizer is removed by extraction and in the processthe porous structure is formed.

The term “electrochemical cell precursor” or “electrolytic cellprecursor” refers to the structure of the electrochemical cell prior tothe addition of the inorganic salt and electrolyte solution. Theprecursor typically comprises (each in precursor form) an anode, acathode, and polymeric matrix or separator. The anode and/or cathode mayeach include a current collector. The polymeric matrix can be fabricatedfrom monomers as described herein. The separator is made of any suitablematerial such as, for example, glass fiber, polyethylene, orpolypropylene.

The term “activation” refers to the placement of an inorganic salt andelectrolyte solvent into the porous portions of an electrochemical cellprecursor. After activation, the electrochemical cell is charged by anexternal energy source prior to use.

The term “electrolytic cell” or “electrochemical cell” refers to acomposite containing an anode, a cathode and an ion-conductingelectrolyte interposed therebetween.

The term “battery” refers to two or more electrochemical cellselectrically interconnected in an appropriate series/parallelarrangement to provide the required operating voltage and currentlevels.

The term “polymeric matrix” refers to an electrolyte compatible materialthat can be formed, for instance, by polymerizing an inorganic ororganic monomer (or partial polymer thereof) and which, when used incombination with the other components of the electrolyte, renders theelectrolyte solid. Suitable polymeric matrices are well known in the artand include matrices formed from organic polymers, inorganic polymers ora mixture of organic polymers with inorganic non-polymeric materials.Preferably, the polymeric matrix is an organic matrix derived from amatrix forming monomer and from partial polymers of a matrix formingmonomer. See, for example, U.S. Pat. Nos. 5,501,921, 5,498,491,5,491,039, 5,489,491, 5,482,795, 5,463,179, 5,419,984, 5,393,621,5,358,620, 5,262,253, 5,346,787, 5,340,669, 5,300,375, 5,294,501,5,262,253, and 4,908,283, which are incorporated herein. Inorganicmonomers are disclosed in U.S. Pat. Nos. 4,247,499, 4,388,385,4,414,607, 4,394,280, 4,432,891, 4,539,276, and 4,557,985, which areincorporated herein.

The matrix forming monomer or partial polymer can be cured or furthercured prior to or after addition of the salt, solvent and, optionally, aviscosifier. For example, a composition comprising requisite amounts ofthe monomer or partial polymer, salt, organic carbonate solvent andviscosifier can be applied to a substrate and then cured. Alternatively,the monomer or partial polymer can be first cured and then dissolved ina suitable volatile solvent. Requisite amounts of the salt, organiccarbonate solvent and viscosifier can then be added. The mixture is thenplaced on a substrate and removal of the volatile solvent would resultin the formation of a polymeric matrix.

Preferably, the polymeric matrix can be formed by a casting processwhich does not require the use of monomers or prepolymers, that is, nocuring is required. A preferred method employs a copolymer ofpolyvinylidene difluoride and hexafluoropropylene dissolved in acetoneor other suitable solvent. Upon casting the solution, the solvent isevaporated to form the polymeric matrix. The solution may be casteddirectly onto a current collector. Alternatively, the solution is castedonto a substrate, such as a carrier web, and after the solvent (e.g.,acetone) is removed, an electrode film is formed thereon.

The term “salt” refers to any salt, for example, an inorganic salt,which is suitable for use in a non-aqueous electrolyte. Representativeexamples of suitable inorganic ion salts are alkali metal salts of lessmobile anions of weak bases having a large anionic radius. Examples ofsuch anions are I⁻, Br⁻, SCN⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, CF₃COO⁻,CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, and the like. Specific examples of suitableinorganic ion salts include LiClO₄, LiSCN, LiBF₄, LiAsF₆, LiCF₃SO₃,LiPF₆, (CF₃SO₂)NLi, (CF₃SO₃)₃CLi, NaSCN, and the like. The inorganic ionsalt preferably contains at least one cation selected from the groupconsisting of Li, Na, Cs, Rb, Ag, Cu, Mg and K.

The term “compatible electrolyte solvent” or “electrolytic solvent,” orin the context of components of the non-aqueous electrolyte, just“solvent,” is a low molecular weight organic solvent added to theelectrolyte and/or the cathode composition, which may also serve thepurpose of solvating the inorganic ion salt. The solvent is anycompatible, relatively non-volatile, aprotic, relatively polar, solvent.Preferably, these materials have boiling points greater than about 85°C. to simplify manufacture and increase the shelf life of theelectrolyte/battery. Typical examples of solvent are mixtures of suchmaterials as dimethyl carbonate, diethyl carbonate, propylene carbonate,ethylene carbonate, methyl ethyl carbonate, gamma-butyrolactone,triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and thelike. When using propylene carbonate based solvents in an electrolyticcell with graphite anodes, a sequestering agent, such as a crown ether,is added in the solvent.

For electrochemical cells where (1) the cathode comprises lithiatedcobalt oxides, lithiated manganese oxides, lithiated nickel oxides,Li_(x)Ni_(1−y)Co_(y)O₂, where x is preferably about 1 and y ispreferably 0.1-0.9, LiNiVO₄, or LiCoVO₄, and (2) the anode comprisescarbon, the electrolytic solvent preferably comprises a mixture ofethylene carbonate and dimethyl carbonate. For electrochemical cellswhere the cathode comprises vanadium oxides, e.g., V₆O₁₃ and the anodeis lithium, the electrolytic solvent preferably comprises a mixture ofpropylene carbonate and triglyme.

The term “organic carbonate” refers to hydrocarbyl carbonate compoundsof no more than about 12 carbon atoms and which do not contain anyhydroxyl groups. For example, the organic carbonate can be non-cycliccarbonates or cyclic aliphatic carbonates. Non-cyclic carbonatesinclude, for example, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, dipropyl carbonate and bis(2-methoxyethyl) carbonate.

Suitable cyclic aliphatic carbonates for use in this invention include1,3-dioxolan-2-one (ethylene carbonate); 4-methyl-1,3-dioxolan-2-one(propylene carbonate); 4,5-dimethyl-1,3-dioxolan-2-one;4-ethyl-1,3-dioxolan-2-one; 4,4-dimethyl-1,3-dioxolan-2-one;4-methyl-5-ethyl-1,3-dioxolan-2-one; 4,5-diethyl-1,3-dioxolan-2-one;4,4-diethyl-1,3-dioxolan-2-one; 1,3-dioxan-2-one;4,4-dimethyl-1,3-dioxan-2-one; 5,5-dimethyl-1,3-dioxan-2-one;5-methyl-1,3-dioxan-2-one; 4-methyl-1,3-dioxan-2-one;5,5-diethyl-1,3-dioxan-2-one; 4,6-dimethyl-1,3-dioxan-2-one;4,4,6-trimethyl-1,3-dioxan-2-one; and spiro(1,3-oxa-2-cyclohexanone-5′,5′,1′,3′-oxa-2′-cyclohexanone).

Several of these cyclic aliphatic carbonates are commercially availablesuch as propylene carbonate and ethylene carbonate. Alternatively, thecyclic aliphatic carbonates can be readily prepared by well knownreactions. For example, reaction of phosgene with a suitablealkane-α,β-diol (dihydroxy alkanes having hydroxyl substituents onadjacent carbon atoms) or an alkane-α,γ-diol (dihydroxy alkanes havinghydroxyl substituents on carbon atoms in a 1,3 relationship) yields an acyclic aliphatic carbonate for use within the scope of this invention.See, for instance, U.S. Pat. No. 4,115,206, which is incorporated hereinby reference in its entirety.

Likewise, the cyclic aliphatic carbonates useful for this invention maybe prepared by transesterification of a suitable alkane-α,β-diol or analkane-α,γ-diol with, e.g., diethyl carbonate under transesterificationconditions. See, for instance, U.S. Pat. Nos. 4,384,115 and 4,423,205which are incorporated herein by reference in their entirety. Additionalsuitable cyclic aliphatic carbonates are disclosed in U.S. Pat. No.4,747,850 which is also incorporated herein by reference in itsentirety.

The term “viscosifier” refers to a suitable viscosifier for solidelectrolytes. Viscosifiers include conventional viscosifiers such asthose known to one of ordinary skill in the art. Suitable viscosifiersinclude film forming agents well known in the art which include, by wayof example, polyethylene oxide, polypropylene oxide, copolymers thereof,and the like, having a number average molecular weight of at least about100,000, polyvinylpyrrolidone, carboxymethylcellulose, and the like.Preferably, the viscosifier is employed in an amount of about 1 to about10 (wt) % and more preferably at about 2.5 (wt) % based on the totalweight of the electrolyte composition.

The term “substrate” refers to any suitable film made of material thatis compatible with the components of the polymer mixture. The substrateserves as the vehicle or base onto which the electrode mixture isapplied. After the solvent has evaporated from the mixture, the polymermatrix is formed. As is apparent, the electrode mixture is applied ontothe surface of the polymer matrix that is not attached to the substrate.Suitable substrates include, for example, paper, e.g, 20 or 24 weightpaper, polyester (MYLAR™), polypropylene, polyethylene films andnon-woven webs.

The term “current collector” refers to any suitable metallic currentcollector. Current collectors in the form of grids are preferred. Eachcurrent collector is also preferably connected to a current collectortab which extends from the edge of the current collector. In batteriescomprising multiple electrochemical cells, the anode tabs are preferablywelded together and connected to a nickel lead. The cathode tabs aresimilarly welded and connected to a lead. External loads can beelectrically connected to the leads. Current collectors and tabs aredescribed in U.S. Pat. No. 4,925,752, 5,011,501, and 5,326,653, whichare incorporated herein.

The composite anode typically comprises a compatible anodic materialwhich is any material which functions as an anode in an electrolyticcell. Such compatible anodic materials are well known in the art andinclude, by way of example, lithium, lithium alloys, such as alloys oflithium with aluminum, mercury, manganese, iron, zinc, intercalationbased anodes such as those employing carbon, tungsten oxides, and thelike. Preferred anodes include lithium intercalation anodes employingcarbon materials such as graphite, cokes, mesocarbons, and the like.Such carbon intercalation based anodes typically include a polymericbinder and extractable plasticizer suitable for forming a bound porouscomposite having a molecular weight of from about 1,000 to 5,000,000.Examples of suitable polymeric binders include EPDM (ethylene propylenediamine termonomer), PVDF (polyvinylidene difluoride), (includingcopolymers thereof), EAA (ethylene acrylic acid copolymer), EVA(ethylene vinyl acetate copolymer), EAA/EVA copolymers, and the like.

Composite anodes preferably comprise a carbon material and a polymericbinder which preferably comprises polymers such as, for example,polyvinylidene difluoride, halogenated hydrocarbon polymers including,for example, poly(vinylidene chloride), poly((dichloro-1,4-phenylene)ethylene), fluorinated urethanes, fluorinated epoxides, fluorinatedacrylics, and copolymers thereof. Porous polymer structures are formedwhen these polymers are first imbued with the plasticizers and thenremoving the plasticizers therefrom. The composite anode can comprisefrom about 5% (wt) to about 50% (wt) preferably from about 8% (wt) toabout 30% (wt) and more preferably from about 10% (wt) to about 20% (wt)of the polymeric binders. Graphite is a preferred carbon material. Thecomposite anode may also include an electron conducting material such ascarbon black.

The composite cathode preferably comprises a compatible cathodicmaterial (i.e., insertion compounds) which is any material whichfunctions as a positive pole in an electrolytic cell. Such compatiblecathodic materials are well known in the art and include, by way ofexample, transition metal oxide materials, sulfides, and selenides,including lithiated compounds thereof. Representative materials includecobalt oxides, manganese oxides, molybdenum oxides, vanadium oxides,sulfides of titanium, molybdenum and niobium, the various chromiumoxides, copper oxides, lithiated cobalt oxides, e.g., LiCoO₂ andLiCoVO₄, lithiated manganese oxides, e.g., LiMn₂O₄, lithiated nickeloxides, e.g., LiNiO₂ and LiNiVO₄, and mixtures thereof. Cathode-activematerial blends of Li_(x)Mn₂O₄ (spinel) is described in U.S. Pat. No.5,429,890 which is incorporated herein. The blends can includeLi_(x)Mn₂O₄ (spinel) and at least one lithiated metal oxide selectedfrom Li_(x)NiO₂ and Li_(x)CoO₂ wherein 0<x≦2. Blends can also includeLi_(y)-α-MnO₂ (0≦y<1) that has a hollandite-type structure and isdescribed in U.S. Pat. No. 5,561,007, which is incorporated herein.

In one preferred embodiment, the compatible cathodic material is mixedwith an electroconductive material including, by way of example,graphite, powdered carbon, powdered nickel, metal particles, conductivepolymers (i.e., characterized by a conjugated network of double bondslike polypyrrole and polyacetylene), and the like, and a polymericbinder to form under pressure a positive cathodic plate. Suitablebinders for use in the cathode have a molecular weight of from about1,000 to 5,000,000.

In one preferred embodiment, the composite cathode is prepared from acomposite cathode paste which comprises from about 35 to 65 (wt) % of acompatible cathodic material; from about 1 to 20 (wt) % of anelectroconductive agent; from about 1 to 20 (wt) % of suitable polymericbinders that may include EPDM (ethylene propylene diene termonomer),PVDF (polyvinylidene difluoride), EAA (ethylene acrylic acid copolymer),EVA (ethylene vinyl acetate copolymer), EAA/EVA copolymers, and thelike. Preferably, the composite cathode can comprise from about 3% (wt)to about 50% (wt), preferably from about 8% (wt) to about 30% (wt), andmore preferably from about 10% (wt) to about 20% (wt) of the polymericbinder.

The composite cathode further comprises from about 0 to about 20 (wt) %of polyethylene oxide having a number average molecular (wt) of at least100,000; from about 10 to 50 (wt) % of solvent comprising a 10:1 to 1:4(w/w) mixture of an organic carbonate and a glyme; and from about 5 (wt)% to about 25 (wt) % of a sold matrix forming monomer or partial polymerthereof. An ion conducting amount of an inorganic ion salt may also beincluded. Generally, the amount of the salt is from about 1 to about 25(wt) %. (All (wt) % are based on the total weight of the cathode.)

After activation, the activated polymeric matrix or separator layertypically comprises from about 5 to about 25 (wt) % of the inorganic ionsalt based on the total weight of the activated polymeric matrix orseparator layer; preferably, from about 10 to 20 (wt) %; and even morepreferably from about 10 to about 15 (wt) %. The percentage of saltdepends on the type of salt and electrolytic solvent employed.

The activated polymeric matrix or separator layer typically comprisesfrom 0 to about 80 (wt) % electrolyte solvent based on the total weightof the layer; preferably from about 60 to about 80 (wt) %; and even morepreferably about 70 (wt) %.

The activated polymeric matrix or separator layer typically comprisesfrom about 5 to about 30 (wt) % of the polymeric matrix or separatormaterial based on the total weight of the layer; preferably from about15 to about 25 (wt) %.

In a preferred embodiment, when employing a polymeric matrix, a smallamount of a film forming agent is added to the polymeric matrix.Suitable film forming agents are well known in the art and include, byway of example, polyethylene oxide, polypropylene oxide, copolymersthereof, and the like, having a numbered average molecular weight of atleast about 100,000. Preferably, the film forming agent is employed inan amount of about 1 to about 10 (wt) % and more preferably at about 2.5(wt) % based on the total weight of the activated polymer matrix.

Methodology

Electrochemical cells are known in the art. See, for example, U.S. Pat.No. 5,300,373, 5,316,556, 5,346,385, 5,262,253, 4,472,487, 4,668,595,5,028,500, 5,584,893, and U.S. patent application Ser. No. 08/630,985entitled “Method and Apparatus For Preparing Electrochemical Cells”filed Apr. 12, 1996, all of which are incorporated herein. The inventivemethod can be adapted to prepare prior art electrochemical devices.

During the conditioning process, conventional charge/discharge cyclingtechniques are employed. For secondary lithium ion cells or batteriescomprising graphite/Li_(x)Mn₂O₄ active materials with a mass balance ofLi_(x)Mn₂O₄:graphite of about 2.9:1, the charging step preferablycomprises charging the freshly assembled electrochemical cell or batteryat a rate of from about 0.25 to about 3 mA per cm², more preferably fromabout 1 to about 2 mA per cm² and maintaining this rate until thepotential of the cell or battery increases to 4.2 volts. In a preferredmethod, the regime comprises (1 ) constant current charge of 1 mA percm² to 4.2V, (2) constant potential float of 4.2V until current falls to0.1 mA per cm², and (3) constant current discharge at 1 mA per cm² to3.0V. Selection of the specific parameters for charge/discharge duringconditioning can be readily determined by the skilled artisan based onthis disclosure.

The following examples illustrate methods of how an electrolytic cellcould be fabricated with the inventive process. Examples 1 and 2describe the process of preparing the anode and cathodes, respectively.Example 3 describes the procedures for fabricating a solid electrolyticcell.

EXAMPLE 1

The anode slurry is prepared as follows: A polymer mixture comprising acopolymer of vinylidene difluoride (VDF) and hexafluoropropylene (HFP)is prepared by mixing 23.3 grams of the copolymer in 100 ml of acetone.The copolymer (ave. melt viscosity 23,000-27,000 poise) is KYNAR FLEX2801™ from Elf Atochem North America, Philadelphia, Pa. The mixture isstirred for about 24 hours in a milling jar available from VWRScientific, San Francisco, Calif., catalogue No. H-04172-00. Thecopolymer functions as a binder for the carbon in the anode.

A graphite mixture is prepared separately by first adding 80 grams ofgraphite into 3.1 grams of carbon black into a solution containing 200grams acetone, 36 grams dibutyl phthalate. A preferred graphite is anatural graphite available as BG35 from Superior Graphite Co., Chicago,Ill. A preferred carbon black is available under the designation SUPERP™ from M. M. M. Carbon, Willbroeck, Belgium. The graphite mixture isthen vigorously mixed in a high shear mixer until a substantiallyhomogeneous blend is formed. A suitable mixer is available from Ross,Model ME100DLX, Hauppauge, N.Y. operating at a high setting (about100,000 RPM) for 30 minutes.

The anode slurry is prepared by mixing the polymer mixture and thegraphite mixture together under low shear conditions to form the anodeslurry wherein the components are well mixed. A portion of the acetoneis allowed to evaporate from the slurry before it was laminated onto acurrent collector comprising a sheet of expanded copper metal. An anodefilm forms when the remaining portion of the acetone evaporates.

EXAMPLE 2

The cathode slurry is prepared as follows: A polymer mixture comprisinga copolymer of vinylidene difluoride (VDF) and hexafluoropropylene (HFP)is prepared by mixing 26.7 grams of the copolymer in 100 grams ofacetone. The copolymer is KYNAR FLEX 2801™. The mixture is stirred forabout 24 hours in a milling jar.

A cathode active material mixture is prepared separately by first adding173.4 grams of a cathode-active material of Li_(x)Mn₂O₄ (spinel)(O<x≦2), 14.7 grams of carbon black (SUPER P™) into a solutioncontaining 333.3 grams acetone, 51.9 grams dibutyl phthalate. Themixture is then vigorously mixed in the a high shear mixer until asubstantially homogeneous blend is formed.

A cathode slurry is prepared by mixing the polymer mixture and thecathode active material mixture together under low shear conditions toform the cathode slurry wherein the components are well mixed. A portionof the acetone is allowed to evaporate from the slurry before it islaminated onto a current collector comprising a sheet of expandedaluminum. A cathode film forms when the remaining portion of the acetoneevaporates.

EXAMPLE 3

A solid electrochemical cell precursor is prepared by positioning apolymeric matrix between the anode and cathode and thereafter laminatingthe structures under moderate pressure and/or temperature (e.g., 130°C.). The lamination causes the polymeric matrix to fuse or anneal withthe binding materials of the anode and cathode. The pressure andtemperature employed will depend on the polymer(s) forming the matrix.The polymeric matrix is formed by casting a slurry comprising acetone,dibutyl phthalate, silanized fumed SiO₂, and a VDF/HFP copolymer onto acarrier web or other substrate and allowing the acetone to evaporate. Nocuring by radiation is required. Preferably, the slurry is mixed underlow shear conditions as not to degrade the copolymer. The anode,cathode, and separator films can be prepared separately or each can befabricated in the form of a long web and stored as individual rolls.Each web can be cut to size as needed. Next the dibutyl phthalate isremoved (e.g. by extraction) from the precursor or plurality ofelectrochemical cell precursors in the case of a battery.

Following removal of the dibutyl phthalate, the precursor is thenpre-packaged in moisture-impermeable material which is described, forexample, in U.S. Pat. No. 5,326,653 which is incorporated herein. Bypre-package is meant that the packaging material is not completelysealed so that gases generated by the cell can be removed. The packagingmaterial employed is any suitable electrically insulating, moistureimpermeable or resistant material such as plastics. Packaging (or finalsealing) can be accomplished by conventional means including, forexample, with adhesives or by shrink-wrapping. At least portions of theanode and cathode tabs remain exposed and extend past the exteriorsurface of the packaging material even after sealing. The pre-packagedprecursor is then activated preferably in an inert (e.g., argon)atmosphere. Next, the electrochemical cell is subject to theconditioning process whereby gases produced by the cell duringcharge/discharge is removed. In one preferred embodiment, the packagecontaining the electrochemical cell has two interconnected chambers. Onechamber houses the cell, and the second chamber is empty. During theconditioning process, the electrochemical cell is heated to an elevatedtemperature. Gas produced during the cycling process enters the secondchamber which is made of flexible, expandable material. After theconditioning process, the conduit connecting the two chambers issevered, leaving two separate packages, one of which contains the cell.The package containing the cell is sealed and is ready for immediate useor storage.

It should be noted that removing 100% of the moisture and other volatileimpurities by the charge/discharge cycle during conditioning isunlikely, so some residual will remain thereby causing secondarygassing. Nevertheless, the elevated temperature improves the kineticsfor all reactions including those associated with gas evolution.

In addition, with the present invention, the cell can be conditioned athigher charge/discharge rates, e.g., 3-5 mA per cm² along with theelevated temperatures to further reduce the conditioning time required.

In another embodiment, the pre-packaged cell is placed into a vacuumchamber which also contains a heating device, e.g. resistive heater.During the conditioning process, gases are continuously removed by avacuum pump. After the conditioning process, the package is sealed.

Experimental

Electrochemical cells fabricated generally in accordance with theprocedures described in Examples 1-3 were conditioned at 23° C. and 45°C. The charge/discharge/charge regime comprised: (1 ) constant currentcharge of 1 mA per cm² to 4.2 V, (2) constant potential float of 4.2 Vuntil current falls to 0.1 mA per cm², and (3) constant currentdischarge at 1 mA per cm² to 3.0 V.

The comparative Electrochemical Voltage Spectroscopy (EVS) voltageprofile data for the two sets of cells conditioned at 23° C. and 45° C.are shown in FIG. 1. EVS is a low rate, high resolution technique forcharacterizing cell performance. For the data shown in FIG. 1, thecomplete charge and discharge cycle took about 40 hours to complete. Thedata presented are for the first charge/discharge cycle. EVS techniquesare further discussed in J. Barker, Electrochimica Acta, Vol. 40, No. 11(1995) 1603-08. As is apparent, there is a clear difference between theinitial voltage responses recorded at the different temperatures. The45° C. profile shows increased irreversible charge consumption (i.e,voltage plateau) around the 3.0-3.2 V range during the charging process.This excess charge consumption does not, however, result in an increasedfirst cycle % inefficiency. Indeed the cells that were conditioned at45° C. actually gave a lower first cycle % loss than those conditionedat ambient temperature.

The 3.2 V voltage plateau is normally associated with creation of thepassivation layer on the graphitic anode material and thus the increasedcharge consumption reflects creation of a more substantialelectrode-electrolyte interface. This, then, may be expected to allowfor improved cycling and temperature storage behavior. The additionalcharge consumption around 3.2 V may also reflect a more efficient gasgeneration mechanism and hence reduce secondary gassing in these cells.

Additional electrochemical cells fabricated generally in accordance withthe procedures described in Examples 1-3 were conditioned at 23° C. and40° C. The same charge/discharge regime was employed.

FIGS. 2A and 2B depict the recharge ratio and capacity respectively vs.cycles for these latter sets of electrochemical cells. The cycling at23° C. at employed the following regime: Charge at 2 mA per cm² untilthe voltage was equal to 4.2V, then the voltage is held at constantuntil the current droped to <0.2 mA per cm². Discharge at 2 mA per cm²until the voltage was equal to 3.0V.

As is apparent, conditioning at elevated temperatures produced cellshaving improved columbic efficiency and improved discharge capacityretention with cycle number. Also, the cells conditioned at 23° C.showed significant secondary gassing during cycling whereas the 40° C.cells showed no secondary gassing because of the improved efficiency ofthe gas evolving reactions.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate the variousmodifications, substitutions, and changes which may be made withoutdeparting from the spirit hereof. The descriptions of the subject matterin this disclosure are illustrative of the invention and are notintended to be construed as limitations upon the scope of the invention.

What is claimed is:
 1. A method of preparing an electrochemical cellthat comprises the steps of: (a) fabricating an electrochemical cell inthe discharged state which comprises (i) an anode comprising anintercalation based carbon material, an (ii) a cathode comprising alithiated cathodic material, and (iii) a polymeric matrix interposedbetween the anode and cathode which comprises an electrolyte solvent andsalt; (b) placing the electrochemical cell in an environment that ismaintained at an elevated temperature of at least 30° C.; (c) chargingand discharging the electrochemical cell; (d) removing gas that isgenerated by the electrochemical cell in step (c); and (e) sealing theelectrochemical cell.
 2. The method of claim 1 wherein the elevatedtemperature ranges from about 35° C. to about 80° C.
 3. The method ofclaim 1 wherein the elevated temperature ranges from about 35° C. toabout 50° C.
 4. The method of claim 1 wherein steps (c) and (d) arerepeated at least one additional time before step (e).
 5. The method ofclaim 1 wherein said anode comprises a second polymeric binder, saidcathode comprises a third polymeric binder, and wherein said polymericmatrix and said first and second polymeric binders comprise a copolymerof vinylidenedifluoride and hexafluoropropylene.
 6. The method of claim1 wherein said lithiated cathodic material comprises lithiated cobaltoxide, lithiated manganese oxide, lithiated nickel oxide and mixturesthereof.
 7. The method of claim 1 wherein the charging is conducted at arate of from about 1 to 4 mA per cm² and discharging is conducted at arate of from about 1 to 4 mA per cm².
 8. The method of claim 1 whereinthe discharging step is accomplished by an external electricalconnection between the anode to the cathode.
 9. The method of claim 1wherein the anode includes an anode current collector that has an anodetab and the cathode includes a cathode current collector that has acathode tab, wherein step (e) comprises surrounding the electrochemicalcell with packaging material to form a package wherein at least aportion of the anode tab and at least a portion of the cathode tabextends past an exterior surface of the packaging material.
 10. Themethod of claim 1 wherein step (e) comprises sealing the electrochemicalcell with electrically insulating and moisture resistant material.
 11. Amethod of preparing an electrochemical cell that comprises the steps of:(a) preparing an anode precursor by forming an anode film comprising ananodic material, a first polymeric binder, and a first plasticizer andthereafter removing said first plasticizer; (b) preparing a cathodeprecursor by forming a cathode film comprising a cathodic material, asecond polymeric binder, and a second plasticizer, and thereafterremoving said second plasticizer; (c) positioning a polymer electrolyteprecursor between said anode film and said cathode film to form anelectrochemical cell precursor and activating the electrochemical cellprecursor to form an electrochemical cell; (d) placing theelectrochemical cell in an environment that is maintained at an elevatedtemperature of at least 30° C.; (e) charging and discharging theelectrochemical cell; (f) removing gas that is generated from theelectrochemical cell in step (e); and (g) sealing the electrochemicalcell.
 12. The method of claim 11 further comprising the step oflaminating said polymeric electrolyte precursor to said anode film andto said cathode film prior to step (d).
 13. The method of claim 11wherein the elevated temperature ranges from about 35° C. to about 80°C.
 14. The method of claim 11 wherein the elevated temperature rangesfrom about 35° C. to about 50° C.
 15. The method of claim 11 whereinsteps (e) and (f) are repeated at least one additional time before step(g).
 16. The method of claim 11 wherein, the charging is conducted at arate of from about 1 to 4 mA per cm² and discharging is conducted at arate of from about 1 to 4 mA per cm².
 17. A method of preparing anelectrochemical cell that comprises the steps of: (a) forming an anodefilm comprising an anodic material, a polymeric binder and a firstplasticizer; (b) forming a cathode film comprising a cathodic material,a polymeric binder and a second plasticizer; (c) forming a polymericlayer comprising a third plasticizer; (d) interposing said polymericlayer between said anode film and said cathode film; (e) removing saidplasticizers to form an electrochemical cell precursor; (f) activatingsaid electrochemical cell precursor to form an electrochemical cell; (g)charging and discharging the electrochemical cell while theelectrochemical cell is maintained at an elevated temperature of atleast 30° C.; (h) removing gas that is generated from theelectrochemical cell during step (f); and (i) sealing theelectrochemical cell.
 18. The method of claim 17 further comprising thestep of fusing said polymeric to said anode film and to said cathodefilm prior to step (e).
 19. The method of claim 17 wherein steps (g) and(h) are repeated at least one additional time before step (i).
 20. Themethod of claim 17 wherein the charging is conducted at a rate of fromabout 1 to 4 mA per cm² and discharging is conducted at a rate of fromabout 1 to 4 mA per cm².
 21. The method of claim 17 wherein the elevatedtemperature ranges from about 35° C. to about 80° C.
 22. The method ofclaim 17 wherein the elevated temperature ranges from about 35° C. toabout 50° C.
 23. An electrochemical cell fabricated by a process asdefined in claim
 1. 24. An electrochemical cell fabricated by a processas defined in claim
 11. 25. An electrochemical cell fabricated by aprocess as defined in claim
 17. 26. A method of preparing anelectrochemical cell that comprises the steps of: (a) fabricating anelectrochemical cell in the discharged state which comprises (i) ananode comprising an intercalation based carbon material, an (ii) acathode comprising a lithiated cathodic material, and (iii) a separatorinterposed between the anode and cathode which comprises an electrolytesolvent and salt; (b) placing the electrochemical cell in an environmentthat is maintained at an elevated temperature of at least 30° C.; (c)charging and discharging the electrochemical cell; (d) removing gas thatis generated by the electrochemical cell in step (c); and (e) sealingthe electrochemical cell.
 27. The method of claim 26 wherein theelevated temperature ranges from about 35° C. to about 80° C.
 28. Themethod of claim 26 wherein the elevated temperature ranges from about35° C. to about 50° C.
 29. The method of claim 26 wherein steps (c) and(d) are repeated at least one additional time before step (e).
 30. Themethod of claim 26 wherein said anode comprises a second polymericbinder, said cathode comprises a third polymeric binder, and whereinsaid first and second polymeric binders comprise a copolymer ofvinylidenedifluoride and hexafluoropropylene.
 31. The method of claim 26wherein said lithiated cathodic material comprises lithiated cobaltoxide, lithiated manganese oxide, lithiated nickel oxide and mixturesthereof.
 32. The method of claim 26 wherein the charging is conducted ata rate of from about 1 to 4 mA per cm² and discharging is conducted at arate of from about 1 to 4 mA per cm².
 33. The method of claim 26 whereinthe discharging step is accomplished by an external electricalconnection between the anode to the cathode.
 34. The method of claim 26wherein the anode includes an anode current collector that has an anodetab and the cathode includes a cathode current collector that has acathode tab, wherein step (e) comprises surrounding the electrochemicalcell with packaging material to form a package wherein at least aportion of the anode tab and at least a portion of the cathode tabextends past an exterior surface of the packaging material.
 35. Themethod of claim 26 wherein step (e) comprises sealing theelectrochemical cell with electrically insulating and moisture resistantmaterial.
 36. The method of claim 26 wherein the separator comprisesmaterial that is selected from the group consisting of glass fiber,polyethylene, and polypropylene.
 37. A method of preparing anelectrochemical cell that comprises the steps of: (a) preparing an anodeprecursor by forming an anode film comprising an anodic material, afirst polymeric binder, and a first plasticizer and thereafter removingsaid first plasticizer; (b) preparing a cathode precursor by forming acathode film comprising a cathodic material, a second polymeric binder,and a second plasticizer, and thereafter removing said secondplasticizer; (c) positioning a separator between said anode film andsaid cathode film to form an electrochemical cell precursor andactivating the electrochemical cell precursor to form an electrochemicalcell; (d) placing the electrochemical cell in an environment that ismaintained at an elevated temperature of at least 30° C.; (e) chargingand discharging the electrochemical cell; (f) removing gas that isgenerated from the electrochemical cell in step (e); and (g) sealing theelectrochemical cell.
 38. The method of claim 37 further comprising thestep of laminating said polymeric electrolyte precursor to said anodefilm and to said cathode film prior to step (d).
 39. The method of claim37 wherein the elevated temperature ranges from about 35° C. to about80° C.
 40. The method of claim 37 wherein the elevated temperatureranges from about 35° C. to about 50° C.
 41. The method of claim 37wherein steps (e) and (f) are repeated at least one additional timebefore step (g).
 42. The method of claim 37 wherein the charging isconducted at a rate of from about 1 to 4 mA per cm² and discharging isconducted at a rate of from about 1 to 4 mA per cm².
 43. The method ofclaim 37 wherein the separator comprises material that is selected fromthe group consisting of glass fiber, polyethylene, and polypropylene.44. An electrochemical cell fabricated by a process as defined in claim26.
 45. An electrochemical cell fabricated by a process as defined inclaim 37.